1. Field of the Invention
The present invention is related to an integrated light guide plate, and more particularly to an integrated light guide plate applied to a backlight module to enhance axial luminosity with microstructures formed on the plate.
2. Description of the Related Art
A backlight module plays a critical role in an active matrix light crystal display. With reference to FIG. 5, a typical backlight module has a light source 50, a light guide plate (LGP) 51, a reflector sheet 52, two diffuser sheets 53a, 53b and two prism sheets 54a, 54b. The light source 50 is formed by either a cold cathode fluorescent lamp (CCFL) or light emitting diodes (LED). A point light source or line light source can be converted into a surface light source through microstructures formed on the LGP 51.
Light-emitting luminance distribution of the LGP 51 could become uneven as a result of the geometrical design of the microstructures. The lower diffuser sheet 53b serves to modify the emitting light into a Lambertian distribution. To improve the axial luminous intensity of a backlight module of a handheld display or personal display with limited luminance, a conventional measure uses the two orthogonally-distributed prism sheets 54a, 54b to collimate the emitting light axially. The collimated light further passes through the upper diffuser sheet 53a to smoothen minor uniformity imperfections or possible Moiré fringes of back light.
According to the manufacturing methods, LGPs (51) can be classified into printed light guide plates and non-printed light guide plates. Printed LGPs are formed by printing a diffusive dot matrix onto plain polymer plates. Non-printed light guide plates are integrally formed with the microstructures by injection molding or hot embossing. Non-printed LGPs usually have a prominent optical performance, in comparison with printed LGPs using dot matrices, since the microstructures of the non-printed LGP do not turn yellowish as a result of prolonged exposure to light. Hence, non-printed LGPs have dominated small-to-medium back light modules (BLMs).
Usually, LGPs have a microstructure taking a form of V-cuts formed on a bottom surface thereof. As such microstructures can vary in density or size along one-dimensional direction, the LGPs having such microstructures are suitable for a CCFL light source. If LED is used as a light source, the distribution of the microstructures has to be varied in two dimensions as the illumination distribution of LEDs is similar to a point light source with a limited emitting angle. Hence, spherical dots and short V-slots become the most common microstructures for LED lit LGPs.
With reference to FIG. 6, a conventional LGP 51 has a microstructure in a form of multiple spherical bumps formed on and protruding from a bottom of the LGP 51, and LEDs are mounted on one edge of the LGP. When light enters the LGP 51, the light is emerged out of the light emission plane of the LGP 51 when subjected to reflection and deflection onto the microstructure. The light propagated by the spherical bumps is reflected and deflected, and then exits along non-specific directions, which might cause an asymmetrical luminance angular distribution.
Certain optical elements such as prisms, convex and concave lens are designed on the edge-lit surface of the LGP to modify the emitting angle and intensity distribution of light sources. With reference to FIG. 7, to improve the aforementioned issue, the LGP 51 has a structure of V-cuts 510 formed on a light incident plane to expand a range of light emitting angle of the light source. In addition, a lower diffusive sheet is often applied on the top of LGP 51 to modify the luminous intensity into a Lambertian distribution.
With further reference to FIG. 7, two luminance angular distributions in vertical and horizontal directions, are measured from the front surface of LGP. A horizontal view angle α is measured in the y-z plane from a surface normal of LGP, and a vertical view angle β is measured in the x-z plane from a surface normal of LGP. The axial luminous intensity is the intensity measured at the surface normal direction in the z direction, i.e. α=β=0.
With reference to FIG. 8, a LGP having a microstructure of spherical bumps on a bottom and prisms on the light incident plane thereof has an asymmetrical luminance angular distribution in vertical angles. The axial luminous intensity is not the highest. The peak intensity appears at vertical angle β=60°. For a personal electronic apparatus with a liquid crystal display, a narrow luminance angular distribution with peak intensity at the normal direction is preferred to increase the up-front brightness.
With reference to FIG. 9, if two orthogonal prism sheets 54a, 54b are mounted on the top of the LGP 51, the luminance angular distribution are collimated as shown in FIG. 10, and the axial luminous intensity increases to attain better personal viewing brightness. However, the peak intensity may appear off the surface normal of backlight module if the luminance angular distribution entering the prism sheets is asymmetrical.
Keeping abreast with the market demand, backlight modules tend to be slim, and light, and head for a goal of low power consumption and low cost. However, as disclosed earlier, conventional LGPs must be additionally equipped with the reflector sheet, the diffuser sheet and prism sheets to acquire more satisfactory emitting luminance. Besides, additional components cause a drop of light efficiency and run counter to the demands of lowering size, weight and cost of the backlight modules.